Active electronic components such as integrated circuits (I.C.'s) on chips of silicon or gallium arsenide, and electro-optic switches or modulators, such as those based on lithium niobate or potassium titanophosphate (KTP) need to have electrical connections and to be mounted in a protective package. While polymeric multilayer substrates are widely used for mounting I.C.'s and providing the necessary interconnective circuitry, ceramic materials, particularly those based on aluminum oxide, are used in applications requiring the highest reliability. In some instances the I.C. is placed inside an alumina module (i.e. chip carrier) which is then mounted with electrical connection (e.g. by soldering) onto a multilayer ceramic substrate on which other components such as additional I.C.'s and/or capacitors and resistors are also attached. In other cases the bare chip is mounted directly onto the multilayer substrate and electrically connected (e.g. by wire bonding) before the complete circuit is encapsulated.
Multilayer ceramic substrates with internally connected metal conductors are well known in the prior art. In general, such structures are formed from ceramic green sheets prepared from suspensions of ceramic powders dispersed in thermoplastic polymer and solvent. Conductors are deposited on some of the green sheets in a pattern, usually by screen printing a paste consisting of a metal powder (preferably one with high electrical conductivity, such as copper), an organic binder and solvent. The sheets with conductors on them may also have via or feed-through holes punched in them to be filled with metal paste, as may be required for interconnections between layers in the final multilayer structure. This structure is then fired to drive off the organic binders and to sinter the ceramic and metal particulates.
The ceramic material used for the substrate must not only have adequate mechanical strength and the ability to dissipate the heat generated during operation of the circuit, but its expansion coefficient must match quite closely the expansion coefficient of the active device, or that of the alumina chip carrier depending on the method of mounting the device. In addition, because many circuits must operate at very high frequencies or signal speeds, the dielectric constant, K, of the ceramic must be as low as possible to minimize coupling between adjacent signal lines (i.e. crosstalk). Furthermore, in order to utilize the high performance of copper conductors within the substrate, the composition of the ceramic must be such that it can be sintered into a dense hermetic dielectric at a temperature below the melting point of Cu (1083.degree. C.) and it must also be resistant to chemical reduction in the atmospheres of low oxygen content that are needed to prevent oxidation of the copper when the substrate is being fired.
Another application for low K dielectrics which can be cofired with copper conductors is in multilayer ceramic capacitors designed for use at high frequencies. A low dielectric constant makes it easier to achieve the precise capacitance tolerance needed for tuner applications. In addition, for a particular capacitance, more dielectric and conductor layers are needed with a low K dielectric because the conductor layers are connected in parallel; this lowers the series resistance of the capacitor leading to sharper resonance (high Q) at high frequencies. Use of copper conductors can also provide high Q because of low conductor resistance.
The present invention addresses the need for improved low K ceramic materials which can be used in ceramic multilayer structures with copper conductors.